Carbohydrates

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  • hemiacetal ketalacetal ketal
  • D L DD D L
  • Carbohydrates

    1. 1. 1Chapter 16Carbohydrates
    2. 2. 21. Types of carbohydratesCarbohydratesconsist of carbon, hydrogen, and oxygen only.are made by plants during photosynthesis.normally have the general formula (CH2O)n.Carbohydrates are made up of saccharide (sugar) units.Monosaccharides consist of a single sugar unit.Disaccharides consist of two sugar units.Oligosaccharides consist of 3-10 sugar units.Polysaccharides consist of long chains, often branched, of sugarunits.
    3. 3. 31. Types of carbohydratesWhat to know from this section:Types of carbohydrates (16.13-14)General formula for a simple sugarSource of sugars (16.16)
    4. 4. 42. MonosaccharidesMonosaccharides have 3-7 carbons.Aldoses have an aldehyde group.Ketoses have a carbonyl group.
    5. 5. 52. MonosaccharidesMonosaccharides are further classified based on thenumber of carbons they contain.triose, tetrose, pentose, hexose, heptoseExamplesA four-carbon monosaccharide with an aldehyde group is analdotetrose.A five-carbon monosaccharide with a ketone group is aketopentose.
    6. 6. 62. MonosaccharidesOnce a monosaccharide has been named as an aldose ora ketose, and the number of carbons has beendesignated, there are still several different isomericforms for each.Each specific monosaccharide has a unique name.A prefix (D- or L-) is added to designate which of two possibleisomeric forms is being referred to.
    7. 7. 72. MonosaccharidesExamples
    8. 8. 82. MonosaccharidesDraw two possible monosaccharide structures for the molecularformula C4H8O4.pencast
    9. 9. 913.4. Reactions--additionAddition of an alcohol to an aldehyde:The product is called a hemiacetal (-OH and –ORattached to the same carbon).Hemiacetals are very reactive.They react with an additional alcohol molecule, losing –OH andadding another –OR.H+
    10. 10. 1013.4. Reactions--additionThe final product is an acetal (2 –OR groups attached toone carbon).hemiacetal acetal
    11. 11. 1113.4. Reactions--additionKetones undergo analogous addition reactions withalcohols.The initial product is a reactive hemiketal (two –R groups, one –OH, and one –OR).An additional –OR group is added to the hemiketal to produce aketal.hemiketal ketal
    12. 12. 1213.4. Reactions--addition
    13. 13. 1313.4. ReactionsHemiacetal, acetal, hemiketal, or ketal?pencastpencast
    14. 14. 1413.4. ReactionsMonosaccharide addition reactions123456]alcoholaldehydeD-glucose]
    15. 15. 15123456Hemiacetal:one –Hone –OHone –ORone -R13.4. ReactionsMonosaccharide addition reactionsThe cyclic form is more stable than the linear form and nofurther oxidation takes place in this case.
    16. 16. 162. MonosaccharidesWhat to know from this sectionDefinitions of aldose and ketose (16.23)Simple sugar naming based on number of carbons (16.25-26)Naming based on aldose/ketose and number of carbons (16.27-28)Identify hemiacetals and hemiketals (16.29)Draw possible monosaccharide structures given molecularformula (16.31-32)
    17. 17. 173. Stereoisomers and stereochemistryStereoisomers havethe same molecular formulas.the same bonding of atoms to one another.Stereoisomers differ in the spatial arrangement of theatoms in the molecule.Enantiomers are stereoisomers that arenonsuperimposable mirror images of each other.A molecule that can exist in enantiomeric forms is called achiral molecule.Simple enantiomers [link—bromochloroiodomethane]
    18. 18. 183. Stereoisomers and stereochemistryA carbon atom that hasfour different groupsbonded to it is called achiral carbon atom.Any molecule containinga chiral carbon can existas a pair of enantiomers.Larger biologicalmolecules often havemore than one chiralcarbon.Glyceraldehyde enantiomers
    19. 19. 193. Stereoisomers and stereochemistryPlane polarized string analogy for plane polarized light.jklhttp://www.chemguide.co.uk/basicorg/isomerism/string4.GIFdemo
    20. 20. 203. Stereoisomers and stereochemistryEnantiomers are also called optical isomers.Enantiomers interact with plain polarized light.They rotate the plane of the light in opposite directions.This interaction with polarized light is called opticalactivity.Optical activity distinguishes the isomers.It is measured in a device called a polarimeter.
    21. 21. 21PolarimeterCompounds that rotate light in a clockwise direction aredextrorotatory, designated by (+) before the angle.Compounds that rotate light in a counterclockwise directionare levorotatory, designated by (-) before the angle.3. Stereoisomers and stereochemistryHome-made polarimeter
    22. 22. 223. Stereoisomers and stereochemistryFischer projections are two-dimensionaldrawings that represent a three-dimensionalmolecule with one or more chiral carbons.The intersection of two lines represents a chiralcarbon.Horizontal lines represent bonds projecting outward.Vertical lines represent bonds projecting backward.
    23. 23. 233. Stereoisomers and stereochemistryBromochlorofluoromethane
    24. 24. 243. Stereoisomers and stereochemistryConventions for drawing monosaccharides as Fischerprojections :The most oxidized carbon is closest to the top.The carbons are numbered from the top.The chiral carbon with the highest number determines the D or Ldesignation.If the OH is to the right, the sugar is D.If the OH is to the left, the sugar is L.Most common sugars are in the D form.
    25. 25. 253. Stereoisomers and stereochemistryDetermine whether each of the followingmonosaccharides is D- or L–.pencast
    26. 26. 263. Stereoisomers and stereochemistryWhat to know from this section:Definitions (bold-face terms)Explanation of plane polarized light, and relationship tostereoisomers (16.37-38)How to identify chiral carbons in molecules (16.45-46)How to interpret Fischer projectionsHow to identify D- and L- sugars (16.44)How to draw the mirror image of a molecule (16.42)
    27. 27. 274. Monosaccharides: glucoseGlucose is an aldohexose (C6H12O6).D- formkjlnmomost oxidized carbon
    28. 28. 284. Monosaccharides: glucoseUnder physiological conditions, glucose exists almostentirely in a cyclic hemiacetal form.The C-5 hydroxyl reacts with the C-1 aldehyde group.C-1 becomes a chiral carbon.
    29. 29. 294. Monosaccharides: glucoseWhen the ringforms, the –OH onC-1 can be below (α-) or above (β-) thering.Isomers that differ inthe arrangement ofbonds around ahemiacetal carbonare called anomers.
    30. 30. 304. Monosaccharides: glucoseThe ring forms are represented as Haworth projections onthe previous slide.Groups on the left of the Fischer projection are above the ring.Groups on the right of the Fischer projection are below the ring.For the cyclic forms of D-sugars, the -CH2OH group is always up.If the –OH on C-1 is cis to the -CH2OH group, it is a β-D-sugar.If the –OH on C-1 is trans to the -CH2OH group, it is an α-D-sugar.
    31. 31. 314. Monosaccharides: glucoseFischer and Haworth projections for D-glucose:
    32. 32. 324. Monosaccharides: fructoseFructose, the sweetest of all sugars, is a ketohexose.Cyclization of D-fructose produces a hemiketal.
    33. 33. 334. Monosaccharides: reducing sugarsBenedict’s test* is used to distinguish between reducingand non-reducing sugars.A reducing sugar can be oxidized.The substance reduced is Cu+2.+ Cu+2  + Cu2O*Benedict’s reagent = a basic buffer solution plus Cu+2 ions
    34. 34. 344. Monosaccharides: reducing sugarsIn general, Benedict’s reagent is used to distinguishbetween aldehydes (e.g., aldoses) and ketones.Ketoses, however, can convert to aldoses in basicsolution.D-fructose enediol D-glucose
    35. 35. 354. MonosaccharidesWhat to know from this section:Be able to relate the open chain form of a monosaccharide toits cyclic form.Understand the relationship between Fischer projections andHaworth projections.Understand the ring-forming reactions that yield hemiacetals orhemiketals.Identify α and β anomers.Know the composition of Benedict’s reagent.Understand what characteristics a monosaccharide needs to bea reducing sugar (react with Benedict’s reagent).Identify the enediol reaction that converts a ketose to analdose.
    36. 36. 365. DisaccharidesIn biological systems, monosaccharides exist in the cyclicform, as hemiacetals or hemiketals.When a hemiacetal reacts with an alcohol, the product is anacetal.When a hemiketal reacts with an alcohol, the product is a ketal.
    37. 37. 375. DisaccharidesA disaccharide is formed when the hemiacetal orhemiketal group on one monosaccharide reacts with oneof the hydroxyl groups on another monosaccharide.The acetal or ketal formed is called a glycoside.The C-O-C bond is called a glycosidic bond.
    38. 38. 385. DisaccharidesMaltose is formed from α-D-glucose and a second D-glucose (α- or β-).In this example, carbon-1 on the α-D-glucose links to carbon-4on the β-D-glucose.The linking oxygen atom is α to (below) the left ring.The connection is called an α(14) glycosidic linkage.
    39. 39. 395. DisaccharidesMaltoseThe hemiacetal hydroxyl group on C-1 will react with Benedict’sreagent, because the ring can open at that point to form an aldehyde(reducing group).The product is named β-maltose because of the position of thishydroxyl group.
    40. 40. 405. DisaccharidesLactose is formed from β-D-galactose and α- or β-D-glucose.In this example, carbon-1 on the β-D-galactose links to carbon-4 onthe β-D-glucose.The linking oxygen atom is β to (above) the galactose ring.The connection is called a β(14) glycosidic linkage.
    41. 41. 415. DisaccharidesThe first step in digestion of lactose is its hydrolysis to re-form galactose and glucose.Glucose is readily metabolized.If the enzyme lactase is not present, lactose can’t behydrolyzed before it is eliminated.This condition is called lactose intolerance.It can be remedied by ingesting lactase when eating lactose-containing foods.An enzyme-catalyzed process makes galactose usable bythe body.Galactosemia is a condition in which one or more of theenzymes is missing.
    42. 42. 425. DisaccharidesSucrose is formed from α-D-glucose and β-D-fructose.Carbon-1 on the glucoselinks to carbon-2 on thefructose (a ketose).The linking oxygen atom is αto (below) the glucose ringand β to (above) thefructose ring..The connection is called an(α1β2) glycosidic linkage.
    43. 43. 435. DisaccharidesIn sucrose, the anomericcarbons of both glucose andfructose are linked.There is no hemiacetal orhemiketal group that can beoxidized.Sucrose is not a reducing sugarand will not react with Benedict’sreagent.
    44. 44. 445. DisaccharidesWhat to know from this section:Identify hemiacetal and hemiketal monosaccharides and theacetal and ketal disaccharides that can be formed from them.Identify glycosidic linkages of various types.Know why maltose and lactose are reducing sugars, whilesucrose is not.Understand the chemical origins of lactose intolerance andgalactosemia.
    45. 45. 456. PolysaccharidesStarch is a heterogeneous mixture of two polymers ofglucose.Amylose (about 20% of plant starch) is a linear polymer of α-D-glucose units connected by α(14) glycosidic bonds.Amylose chains can contain up to 4,000 glucose units.Amylose coils into a helix that repeats every six glucose units.
    46. 46. 466. PolysaccharidesStarch is a heterogeneous mixture of two polymers ofglucose.Amylopectin consists of an amylose backbone with chains of [α(14)glycoside-bonded] glucose units branching off the C-6 hydroxylgroups by α(16) glycosidic bonds.Each of the many branches contains 20-25 glucose units.
    47. 47. 476. PolysaccharidesDigestion of starch:Two enzymes are produced in the pancreas and the salivaryglands.α-Amylase cleaves the α(14) glycosidic bonds randomly alongthe amylose chain to make shorter polysaccharides.β-Amylase sequentially cleaves pairs of glucose units (thedisaccharide maltose) from the reducing ends of amylosechains.
    48. 48. 486. PolysaccharidesGlycogen is the main glucose storage molecule.Its structure is identical to amylopectin’s, exceptthe branches are shorter,there are many more branches.amylopectinglycogen
    49. 49. 496. PolysaccharidesCellulose is a straight-chain polymer of β-D-glucose unitslinked by β(14) glycosidic bonds.Typical cellulose molecules contain about 3,000 glucose units.Cellulose is part of the structure of plant cell walls.Humans (and all but a few animals) can’t digest cellulose becausethey lack the enzyme cellulase that hydrolyzes the β(14) glycosidicbond.
    50. 50. 506. PolysaccharidesWhat to know from this section:For amylose, amylopectin, glycogen, and cellulose:the arrangement of the glucose unitsthe type(s) of linkagesThe functions of α-amylase and β-amylaseWhy humans can’t digest cellulose

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